In recent years, realization of an ultraviolet light-emitting diode has been very strongly desired in the industrial field as a breakthrough following the blue light-emitting diode. That is because a shorter light emission wavelength leads to expectation of various new functions such as enhancement of optical recording density and excitation of a white light-emitting fluorescent material.
As a promising candidate for a material enabling t to realize the ultraviolet light-emitting semiconductor device, ZnO has heretofore attracted attention and has been studied extensively all over the world.    Non-patent Document 1: Ohtomo et al., APL72, 2466 (1998)